Public telecommunications systems include subscribers who are coupled to a telecommunications network with a twisted pair wire loop commonly known as a subscriber loop. Transmission systems based on local subscriber loops are generally called Digital Subscriber Lines (DSL). DSL signaling is used to convey digital data over existing twisted-pair copper telephone lines connecting the telephone company central office (CO) to residential subscribers, and conventional DSL data modems are designed to provide service to a certain percentage of residential customers at a prescribed data rate. In general, telephone lines employ twisted pairs of wire in order to mitigate crosstalk that can occur between tightly packed pairs carrying unrelated information streams.
An example of conventional use of DSL techniques is the Asymmetrical Digital Subscriber Line (ADSL) modulation method for the telephone loop that has been defined by standards bodies as a communications system specification that provides a low-rate data stream from the residence to the CO (upstream), and a high-rate data stream from the telephone company CO to the residence (downstream) over the same single pair. In present implementations of this modulation technique, the two directions of information flow are disjoint in the frequency domain, and consequently, it is comparatively simple to protect the receiving means at each end of the path from the collocated transmitting means.
ADSL modems are typically installed in pairs, with one of the modems installed in a home and the other in the telephone company's CO servicing that home. The pair of ADSL modems are connected to the opposite ends of the same twisted-pair and each modem can only communicate with the modem at the other end of the twisted pair. The CO will have a direct connection from its ADSL modem to the service provided (e.g., movies, Internet, etc.). An ADSL modem operates at frequencies higher than the voice-band frequencies.
Another high speed data service is known as a Symmetrical Digital Subscriber Line (SDSL), wherein, unlike an ADSL service, the information rate is intended to be equal in both the upstream and downstream directions. Currently, data speeds as high as 1.5 Mbps in each direction are common, again employing only a single pair of wires. The operating range of an SDSL circuit is, however, limited to about 10,000 feet. Further, with current SDSL techniques, the send and receive frequency spectra completely overlap. Thus, the receiver at each end must not only equalize for channel dispersion introduced by the twisted pair, but must also discriminate against the collocated transmitter signal through a process known as echo cancellation. Because of the fast data rates desired, the available bandwidth of the twisted pair must be vigorously exploited.
Modem spectral components as high as 1 MHz are commonly employed, and at these higher frequencies, path losses over 10,000 feet can be expected to be more than 70 dB (roughly one part in three thousand). Naturally then, to effectively discriminate the far end signal from the near end transmitted signal, the near end receiver echo canceller must be exquisitely adjusted. Currently, this adjustment is not only done during the SDSL modem-to-modem start-up phase, but also more gradually on a continuous basis.
Inevitably, DSL transmission through a wire twisted pair introduces distortions, and is limited by such things as loop loss, the noise environment, and modem transceiver technology. The impairments that must be tolerated increase with loop length, and bandwidth employed.
For the purposes of network maintenance and assurance of quality of service, it is necessary for the provider of network services to be able to monitor the path established between connected users at various points throughout the network. One such point is the loop connecting the serving modem to the subscriber modem. Abrupt connection of monitoring means, without disruption, to the loop, where the transmission methods are analog, is not a difficult challenge when narrow bandwidth services are transported such as Plain Old Telephone Service (POTS) or comparatively slow voice band modem service. Even well-established high speed data services such as T1, which operates at 1.544 Mbps unidirectionally on any one pair, are quite simple to monitor, or sample, at an analog point. This is a result of rudimentary encoding techniques, short distances which result in only modest signal level losses, and unidirectional transmissions.
In an attempt to increase the utilization of presently installed twisted pair loops by employing bidirectional information flow, faster data rates, and longer distances between regeneration devices, DSL services have adopted modems employing considerably more complicated encoding techniques, and massively more complicated receiving means. One negative ramification of this is that the permissible degree to which a selectively applied monitor device can alter the apparent characteristics of a transmission path, is reduced.
Extensions of conventional monitor access techniques have been successful in avoiding data flow disruptions of the currently employed variants of the ADSL services. The same enhanced techniques, however, have been disruptive to a significant percentage of SDSL circuits. Further reductions of the inevitable loading imposed as a result of the selective application of monitor circuitry to a loop would reduce the incidence of disruption, at the expense of circuit complexity, and thus, cost. Another approach which eliminates changes in loop characteristics, is to permanently attach individual monitoring, or sampling means, to each individual loop. This of course can also prove to be prohibitively bulky, complex, and thus, costly.
To understand the advantages of the present invention's gradual monitor access (GMA) technique over more conventional approaches, a very cursory comparison of the generally deployed ADSL and SDSL communication technique is provided below, particularly with regard to how transmission limitations are inherent with twisted pair media.
As stated above, the objective of a DSL service is to move data reliably at as fast a rate as possible within the limitations of the utilized twisted pair media between two locations. Two impairments which are immutably coupled to the geometry of, and materials used to build, the twisted pair, are attenuation and dispersion. Attenuation is the characteristic reduction of amplitude observed, as a result of energy loss, when a signal propagates through a twisted pair. Ultimately, if the utilized pair is allowed to become arbitrarily long, the receiver will no longer be able to distinguish between transmitted symbol states, as a result of diminished signal level relative to the inescapable thermal noise level. Thus, as stated above, there is a line length limitation.
Another consequence of using a physical medium to move data, is that a finite amount of time is consumed to allow the symbols representing the data to transfer from one end of the medium, to the other. Variability in this speed of propagation, when it is a function of signal frequency, is referred to as dispersion. Dispersion is evidenced at the receiver by symbol distortion, and the spreading over time of the effect of a particular symbol to such a degree that adjacent symbols in time are also affected. At fast symboling rates, this (Inter Symbol Interference (“ISI”) can be so severe that detection of the desired symbol state by the receiver is impossible.
A third impairment, echo, for practical reasons, is also incurable. Twisted pair media nonuniformities, such as wire gauge changes, bridge taps, fabrication tolerances, imperfect terminations, and poor splices, create return reflections of propagating signals, which then appear as echoes to the sender, as well as delayed ghost signals to the receiver. Since receiver and transmitter at each end are connected together to the same twisted pair, echoes and ghosts created by progenitor symbols from either end will challenge the receiver at both ends. Because of the distributed nature of the various loop nonuniformities over the loop length, which create these partial reflections, the echoes and ghosts can arrive many symbol time intervals delayed from the progenitor symbol arrival at the intended receiver. Echoes and ghosts can thusly, create non-adjacent ISI, which further impairs the ability of the receiver to distinguish symbol states.
Both ADSL and SDSL modems mitigate attenuation through the use of gain, consistent with the receiver noise levels which of course are also magnified by the addition of gain. Both modem types use an equalizer to cancel out, or deconvolve the dispersion and ghosts. An equalizer is in essence a long ideal signal delay algorithm which has many uniformly spaced incremental adjustable weight taps, the outputs of which are summed to form the equalizer output. Through an iterative process of comparison of received symbol shape as seen at the equalizer output, and the possible ideal symbol shapes, the equalizer taps are adjusted to optimize the shape, and thus recognizability, of the exiting symbols. For later reference, present ADSL modems train the receiver equalizer once, during end to end synchronization. A modest margin of performance is reserved to accommodate small transmission characteristic shifts over time, without necessitating retraining and resynchronization.
SDSL modems on the other hand, continue to adapt to transmission characteristic shifts even after the rapid initial training interval, as long as the changes accrue slowly. Thus, in theory, the SDSL modem could avoid down time for retraining caused by transmission path characteristic shifts. In practice, however, another design choice made in the SDSL architecture, namely, to completely overlap the sending and received frequency spectrum, creates a sensitivity to transmission path variations which overshadows the presently apparent advantage.
For both ADSL and SDSL modems, one twisted pair is used for transmission in both directions. Thus, if there are any path echoes, which are inevitable, the receiver at each end must not only compensate for transmission impairments sustained by a distant end signal, but also reject adequately echoes of the collocated sender. Current ADSL modems do not overlap the sending and receiving frequency spectrums, and adequate rejection of the near end sender is possible through the use of analog and/or digital filtering to split the send and receive frequency bands.
Fortuitously, the effectiveness of this filtering is essentially independent of transmission path variations. In contrast, because SDSL modems completely overlap the send and receive spectrum, it is not possible to use frequency band splitting to immunize the receiver of interest from disruption by the associated transmitter. In order to make such as scheme work, another way to discard the near end sender signal from the gross received signal must be employed.
One such approach is to use an echo canceller, which is identical in principle to the equalizer mentioned earlier, except that the equalizer input signal is a sample of the collocated send signal, and tap weights, or coefficients, are adjusted to create an output, which is a close replica of the sender echoes contaminating the desired receive signal. When the replica is subtracted from the gross received signal, a resultant, essentially free of near end sender influence, is produced. The echo canceller coefficients are iteratively determined by comparing the known send symbols with the corrected received signal, and minimizing any correlation.
In particular, U.S. Pat. No. 5,617,450 (Kakuishi et al) disclose using an echo canceller for carrying out an echo canceling operation in a DSL interface unit, and U.S. Pat. No. 6,181,791 (Murphy) disclose reducing local echo in a communication system, particularly an ADSL. Further, U.S. Pat. No. 5,329,586 (Agazzi) disclose achieving distributed lookup table nonlinear echo cancelation with improved convergence.
The difficulty in relying principally on an echo cancellation technique, however, is that it is exquisitely sensitive to transmission path variations, particularly when the path length approaches the allowed maximum, and the disturbance to the path occurs close to one of the modems. Again, although slowly accruing changes are tolerated as a consequence of slow but continual adaptation, even tiny abrupt changes can cause loss of synchronization and subsequent retraining.
One measure of transmission path variations is to examine the value of a characteristic impedance, as a function of position along the length of the subject twisted pair. Twisted pair transmission paths used for DSL service typically display a characteristic impedance of, for example, roughly 100 ohms. Assuming the transmission path is terminated in its characteristic impedance at each end so that any energy leaving the path is totally absorbed by the terminations, and consequently no echoes are created, the apparent circuit impedance at any point along the path is one half of the characteristic impedance, or in this case, approximately 50 ohms.
Experiments conducted by the Applicant have shown that for path lengths approaching the specified maximum, for SDSL services, an abrupt change in this apparent impedance of only one part in a thousand is enough to consistently result in loss of synchronization. Conventional monitor access techniques cause small but abrupt circuit impedance changes, which consequently, cause an unacceptably high occurrence rate of synchronization failure when used randomly over the population of SDSL circuits. A design response to this difficulty is to raise the impedance of the monitor circuit to such a value, that abruptly connecting it to the subject SDSL circuit, causes less than a one part per thousand change in circuit impedance. However, practical limitations imposed by parasitic capacitance of the monitor circuit and connecting means to the subject twisted pair carrying DSL, amplifier and thermal noise levels, required bandwidth, and increased cost, are quickly reached.
Thus, there have been difficulties experienced during field trials in monitor access application while attempting to gain monitor access to certain types of DSL circuits, such as SDSL, without disruption. ISDN (Integrated Services Digital Network) modems also rely primarily on echo cancellation to function, and are thus, similarly sensitive to abrupt path characteristic changes, although to a lesser degree than the essentially similar SDSL modems, as a result of much lower rate and bandwidth requirements. As stated above, abrupt changes in media parameters, for this type of data communications equipment, cause unacceptable information rate or accuracy impairments, an in some cases, cessation of information flow.
However, in order to ensure quality of service, it is often necessary to unobtrusively monitor the progress of communications over the transmission media by connection to the media itself. Unless the monitor facility is permanently in place, the introduction or removal of the monitor, to some degree, disturbs the transmission parameters of the media. In order to minimize disruption of information flow, the loading of the monitor device should be reduced as much a possible, to the point that it can be abruptly applied without harm. However, practical limits prevent the economic realization of a shared monitor with sufficiently large bandwidth and low internal noise, simultaneously with sufficiently slight loading, to consistently permit abrupt application upon, or removal from all presently employed conventional circuits. A monitor device that would otherwise be unacceptable when abruptly applied, can be made to connect and disconnect without disruption if the ultimate loading is applied and removed in a sufficiently slow manner.
Further, although monitor access can be performed in conventional telephone data transmission systems, as disclosed in U.S. Pat. No. 5,581,228 (Cadieux et al), it has been difficult to obtain, without disruption, in DSL's.
For example, to obtain a sample of the voltage waveform appearing between the tip and ring lead of the selected loop, at the point of access, without affecting any of the twisted pair transmission characteristics, or disruption of ongoing communications, an infinite input impedance buffer amplifier can be employed with accompanying means to select and connect to the target loop, of vanishingly small physical dimensions. Alternatively, a less than ideal voltage monitor device could be permanently attached to each loop which could be potentially selected. Obviously, neither case is practical.
In contrast, a directional coupler can also be used as a sampling device, with the added benefit that the energy contribution of each modem can be sampled and substantially separated. This directivity is a predictable consequence of the characteristics of transmission lines which have been intentionally coupled by placement in close proximity to one another, over a length significant relative to the wavelength of the lowest frequency of interest. However, for the frequency band of immediate interest, which is roughly 30 kHz to 1 MHz, the dimensions of a true coupled line directional coupler would be ponderous. Thus, an approximation which would be more compact is desired. As one of ordinary skill in the art would recognize, such an approximation can be constructed using lumped circuit elements, within any arbitrary accuracy over a given bandwidth, as a function of how finely the constituent inductive and capacitive elements are divided. This approach however, also poses practical difficulties. Specifically, a lumped element directional coupler involves many more parts than a simple bridging monitor, some of which (the inductors or transformers), must be introduced in series with the tip and ring leads, not simply tapped to them. The abrupt insertion or removal of such a coupler will create severe data circuit disruptions.
One way around this is to introduce a directional coupler into each pair of interest on a permanent basis, however, such a solution would result in unacceptable equipment volumes and cost. Thus, several practical matters make the application of a conventional directional coupler prohibitive for the purpose at hand.
Therefore, the Applicant has determined that what is needed is a way to slowly apply and remove the coupling. One approach is to mechanically move the coupling transmission line section from a distance, to within close proximity of the data circuit pair to be examined. However, the implementation of this approach, is quite impractical at frequencies below roughly 1 Megahertz (i.e., wavelengths exceeding 200 meters in the twisted pair), within which all of the modems, considered thus far, operate.
A mechanical means of gradually applying a lumped directional coupler is imaginable, but extremely intricate, wherein the inductive elements could have cores with openings which would allow the slow and separate introduction of the subject tip and ring leads, and the required capacitive elements could be achieved between approaching conductive structures of sufficient area.